This invention relates to methods for chemically or electrochemically removing material from metal substrates. More particularly, this invention relates to methods for preparing solutions for selectively removing material from metal substrates.
A variety of coatings are used to provide oxidation resistance and thermal barrier properties to metal articles, such as turbine engine components. Current coatings used on components in gas turbine hot sections, such as blades, nozzles, combustors, and transition pieces, generally belong to one of two classes: diffusion coatings or overlay coatings. State-of-the-art diffusion coatings are generally formed of aluminide-type alloys, such as nickel-aluminide, platinum-aluminide, or nickel-platinum-aluminide. Overlay coatings typically have the composition MCrAl(X), where M is an element from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Diffusion coatings are formed by depositing constituent components of the coating, and reacting those components with elements from the underlying substrate, to form the coating by high temperature diffusion. In contrast, overlay coatings are generally deposited intact, without reaction with the underlying substrate.
When articles such as gas turbines are serviced, accumulations of foreign matter such as dirt and mineral deposits often need to be removed from the surface of components. In addition, protective coatings usually must be removed to permit inspection and possible repair of the underlying substrate, followed by re-coating. Removal of the coatings and foreign matter is typically carried out by immersing the component in at least one treatment solution. A variety of treatment techniques are currently available for removing different types of coatings and foreign matter from metal substrates. The techniques usually must exhibit a considerable amount of selectivity. In other words, they must remove only the intended materials, while generally preserving the article""s desired structures.
One example of a particular treatment technique to remove metallic coatings and foreign matter is chemical etching. In such a process, the article is submerged in an aqueous chemical etchant. Foreign matter and the metallic coating on the article surface are then dissolved as a result of reaction with the etchant.
While many stripping techniques are very useful for a variety of applications, they may not always include the features needed in specialized situations. As an example, many forms of chemical etching are generally nonselective, and can result in undesirable loss of the substrate material. This material loss often leads to changes in critical dimensions, e.g., turbine airfoil wall thickness or cooling hole diameter. The material loss can also lead to structural degradation of the substrate alloy, e.g., by way of intergranular attack. Moreover, chemical etching can result in the stripping of coatings from internal passages in the article, which is often undesirable.
Masking techniques can be used to protect portions of a component""s structure from the effects of treatment solutions. For example, masking is often used to protect the internal cooling passages and holes in turbine engine components. However, masking and the subsequent removal of the masks can be time-consuming and labor-intensive, detracting from the efficiency of a repair process.
In the case of removing metallic coatings, electrochemical stripping processes overcome some of the disadvantages inherent in conventional techniques such as chemical etching. For example, a U.S. patent application of Bin Wei et al, Ser. No. 09/420,059, describes a useful electrochemical stripping process. In general, the process selectively removes metallic coatings from the external sections of a metallic article, such as a turbine component. The process employs an electrolytic solution based on various compounds, such as organic and inorganic salt/solvent systems. Examples of electrolytic systems are ammonium chloride/ethylene glycol, and aqueous sodium chloride. An advantage of this type of process in theory is that coatings on internal passageways of the component remain unaffected by the action of the stripping agentxe2x80x94even when they have not been masked. However, improvements are often desirable in practical applications. For example, ammonium chloride-type electrolytes can sometimes damage the base metal of an article. Moreover, some of the electrochemical stripping processes do not provide a wide enough xe2x80x9cprocess windowxe2x80x9d for efficient commercial operation, and some exhibit low selectivity due to chemical attack of coatings on internal channels. In some cases, the time period between complete stripping of the coating and the occurrence of significant damage to the substrate may be too short.
These shortcomings are addressed in the U.S. patent application of Bin Wei et al., Ser. No. 09/682,620. In part, an electrochemical method described therein includes the step of immersing the substrate in an aqueous composition comprising an acid having the formula HxAF6, where xe2x80x9cAxe2x80x9d is at least one of Si, Ge, Ti, Zr, Al, and Ga; and x is in the range from about 1 to about 6. This treatment solution composition provides significantly enhanced selectivity relative to other solutions, along with a sufficiently robust processing window for commercial stripping and cleaning processes. In some cases, however, difficulties arise in consistently maintaining the selectivity of the treatment solution comprising HxAF6. Therefore, there remains a need to provide improved treatment solutions and methods for selectively cleaning and stripping materials from metal articles.
The present invention provides several embodiments that address this need. One embodiment is a method for preparing a solution for treating an article, herein referred to as xe2x80x9cthe solution preparation methodxe2x80x9d. The solution preparation method comprises providing a quantity of treatment solution, the treatment solution comprising an acid having the formula HxAF6, wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al, and Ga, and x is in the range from about 1 to about 6; determining a concentration of free acid contaminant in the treatment solution; and removing the concentration of free acid contaminant.
A second embodiment is a method for treating an article to selectively remove unwanted material. The method comprises providing an article, the article comprising unwanted material; preparing a treatment solution according to the aforementioned solution preparation method; and exposing the article to the treatment solution.
A third embodiment is treatment solution made by the aforementioned solution preparation method.